The present invention relates to a brushless motor using an oil-impregnated bearing.
Outer rotor type brushless motors are used as spindle motors for rotationally driving color wheels in projectors, polygon mirrors in printers and the like. In a motor of this sort, high precision rotational performance is required, and while primarily rolling bearings were conventionally used for bearings therein, problems with rotation noise and difficulties in achieving cost reductions has meant that oil-impregnated bearings, which are made of a porous material impregnated with lubricating oil at the interior, have recently come into use.
In terms of oil-impregnated bearings of the sort described above, Japanese Patent Laid-Open Publication No. 2006-292161 describes a double bearing unit comprising an inner part made from a compacted sintered metal body having bearing parts at both ends, and an outer part, this inner part being concentrically press fit at the inner circumferential face of the outer part. A compacted sintered metal material having a lower density than the inner part is used for the outer part in this bearing unit.
With the bearing unit described in Japanese Patent Laid-Open Publication No. 2006-292161, a larger amount of lubricating oil can be retained in the outer part than in the inner part and a good density gradient is formed by way of the capillary action of the inner part, which has a higher density than the outer part, such that lubricating oil will tend to be gradually drawn into the inner part from the outer part. Consequently, a bearing unit can be produced with which there is little vibration, such that it is very quiet and has an extended life.
Furthermore, Japanese Patent Laid-Open Publication No. 2009-85355 describes an oil-impregnated bearing mechanism provided with an oil-impregnated double bearing comprising an inner bearing made from a porous material containing a lubricant and having an inner circumferential face serving as a bearing face that rotatably supports a shaft, and an outer bearing made from a porous material containing a lubricant, which has a higher porosity than the inner bearing. In this oil-impregnated bearing mechanism, a communicating groove, which communicates with the exterior, is formed between the outer circumferential face of the inner bearing and the inner circumferential face of the outer bearing, using a portion of the outer circumferential face of the inner bearing and a portion of the inner circumferential face of the outer bearing.
With the oil-impregnated bearing mechanism described in Japanese Patent Laid-Open Publication No. 2009-85355, a communicating groove is formed between the outer circumferential face of the inner bearing and the inner circumferential face of the outer bearing, such that lubricant exuded from the inner bearing and the outer bearing can be retained in this communicating groove, allowing an oil-impregnated bearing mechanism to be produced which has an extended life.
With the oil-impregnated double bearings described in Japanese Patent Laid-Open Publication No. 2006-292161 and Japanese Patent Laid-Open Publication No. 2009-85355, if the outer part (outer bearing) has a lower density than the inner part (inner bearing), it is possible to maintain the inner diameter precision of the inner part when the inner part is press fit into the outer part.
However, when a motor is configured using the oil-impregnated double bearings described in Japanese Patent Laid-Open Publication No. 2006-292161 and Japanese Patent Laid-Open Publication No. 2009-85355, there are problems insomuch as, when the oil-impregnated double bearing is press fit into a housing, it is difficult to maintain the inner diameter precision of the inner part that directly supports the shaft, and thus it is difficult to achieve high precision rotational performance.